Paragraph #1: This photovoltaic field has been dominated by solid state inorganic solar cells based on doped-Si. A new family of solar cells based on nanocrystalline and conducting polymer films is emerging, some of which show high solar conversion efficiencies, rivaling that of traditional Si devices.
Paragraph #2: Dye sensitized nanocrystalline solar cells (DSC) are based on the operating principle of charge separation between wide band gap oxide layers, a monolayer of charge transfer dye, and an electrolytic redox solution. Overall the DSC creates an electrical potential based on the difference in the Fermi level of electron in the oxide layer and the electrolyte solution, which out any permanent chemical reactions.
Paragraph#3: TiO2 layers are typically deposited by screen printing onto a conductive glass surface. Typical geometries of deposited oxide layers are 5-20 um thick and are have porosity between 50-65%.
Paragraph#4: New developments in DSCs are the use of a wide band gap nanocrystalline n-type semiconductor as an electron acceptor. The charge neutrality on the dye is restored by a p-type semiconductor acting as an electrolyte; cells with this advancement have shown efficiecies of over 10%.
Paragraph#5: DSCs have interesting history and corrsponance with photography and photo-electrochemistry. TiO2 is now playing the same role as silver halides, which also have large band gaps on the order of 2.7-3.2 eV, did for photography.
Paragraph#6: The first sensitization of a photo-electrode occurred shortly after 1873. It took until the 1960s to realize that the excitation of a dye molecule injecting electrons into the conduction band of an n-type semiconductor.
Paragraph#7: Titanium dioxide became the semiconductor of choice for a handful of reasons. Using a tri(bpy)carboxylate-Ru dye to chemisorb onto the surface of the a 7.1% efficiency DSC was announced in 1991. Progress has continued since with efficiency over 10% today.
Paragraph#8: The ideal photosensitizer would absorb all light below 920nm and contain contain carboxylate or phosphonate groups to graft to semiconductor oxide surface. Redox potential must be low to be regenerate from electron donation from the redox electrolyte and be stable enough to continuously undergo this reaction for about 20 years.
Paragraph#9: Research in dye chemistry focuses on absorption over the entire visible range. Optical absorption of a single monolayer is weak, unless the oxide layer becomes highly porous.
Paragraph#10: Ru and Os polypyridyl complexes show the most promise for dyes, i.e. the N3 dye. The max absorption are 518 and 380nm, emits at 750nm with the lifetime being 60ns.
Paragraph#11: N3 was surpassed in 1993 by the “black dye” a trpy complex, efficiency excedded 10.4%, and furthermore a dual system of N3 and the black dye were even better.
Paragraph#12: The black dye absorbs 100nm further into the IR than N3. However, both dyes show large overlap in the majority of absorption spectrum.
Paragraph#13: Molecular engineering of Ru complexes has increased light harvesting in the 700-900nm region. The goal is to mimic GaAs optical features, which would increase overall efficiency to 15%.
Paragraph#14: DSC operation is insensitive to temperature changes, however, competing Si solar cells see drastic decrease under raised temperature up to 60C.
Paragraph#15: Porphyrins and phthalocyanines are also of particular attention to organic dyes. Porphyrins generally lack absorption near the IR, while phthalocyanines have LUMO levels too low for electron transfer into the TiO2 conduction band.
Paragraph#16: Coumarine and polyene sensitizers have achieved up to 7.7% efficiency under full sunlight.
Paragraph#17: The combination of two complementary dyes to achieve broad visible absorption is another successful strategy. Many dye combinations need to be tested for this type of system.
Paragraph#18: Semiconductor Quantum dots show great promise due to very high extinction coefficients. These types of sensitizers are most attractive for solid-state heterojunction devices.
Paragraph#19: Mesoporous electrodes are very different than the same material in compact forms. The charge transport mechanism is still under debate in these systems, and percolation processes are under intense scrutiny. Intensity modulated impedance spectroscopy has been an effective tool to address these problems.
Paragraph#20: Future research on mesopourous electrodes will be based on spatial organization rather than random assembly. Nanorod and Nanotubes hold the most promise, this would facilitate pore diffusion and allow the junction to form with better control.
Paragraph#21: High surface area of nanocrystalline solar cells is important for performance enhancement. Several computational studies have been executed using classical force field and first principle density functional calculations to model adsorption of dyes onto TiO2 surface planes.
Paragraph#22: Synthetic efforts for sensitizers should focus on keeping one-directional flow of electrons from the redox electrolyte through the junction and into the oxide layer after photo-excitation of the sensitizer. New dyes should also form a tightly packed monolayer to block dark current.
Paragraph#23: Recent progress was made in controlling self-assembly of dye by adding guanidinium thiocyante to the electrolyte solution. This approach led to an efficiency of 10.6%.
Paragraph#24: A photovoltaic device must be stable for 20 years without performance degradation. A constituents must be under intense scrutiny, since the conductive glass and TiO2 films are essentially required four other components have received the most investigation.
Paragraph#25: The N3 dye is stable in elevated temperatures and can undergo up to 108 redox cycles without considerable loss of performance. Outstanding stability can be attributed to charge injection in a femtosecond time-scale. Short time frames do not allow other chemical mutations to occur.
Paragraph#26: Long term illumination of redox electrolytes like cyclic carbonates, such as propylene or ethylene carbonate, which were found to undergo thermally activated decarboxylation in the presence of TiO2 rendering them unsuitable for reliable use.
Paragraph#27: Highly polar nonvolatile solvents have been found to be ample replacements, such as methoxypropionitrile (MPN). MPN based electrolyte in conjugation with a surfactant ruthenium dye recently passed 1000h stability at 80C.
Paragraph#28: Room temperature molten salts have attractive stability and some based on imidazolium iodides have shown efficiency up to 6%.
Paragraph#29: Long-term light soaking has seen significant progress over the past few years. A recent 12,000h test under full illumination has recently passed, where traditional amorphous silicon fails due to the Stabler-Wronski effect.
Paragraph#30: The replacement of redox electrolyte by p-type semiconductor is a promising alternative to deal with sealing problems.
Paragraph#31: The sensitizing dye does not provide a conducting functionality; therefore it must be in direct contact with both conducting phases. It is not completely evident that the penetrating solid interface will allow contact intimate enough for reasonable charge transfer to occur.
Paragraph#32: The system in Paragraphs 29 and 30 have been improved since 1998. Highest efficiency has been seen at 3.8% by Meng et. al with Cu(I) as a hole conductor instead of a liquid electrolyte.
Paragraph#33: Dye-Sensitized nanocystalline photovoltaic cells have shown great promise in competing with solid-state junction devices.
My research topic is going to be advances in dye sensitized solar cells.
Graetzel, M. Journal of Photochemistry and Photobiology C. 4. (2003). 145-153.
http://dx.doi.org/10.1016/S1389-5567(03)00026-1
Assignment #2
Chem 767
Paragraph #1: This photovoltaic field has been dominated by solid state inorganic solar cells based on doped-Si. A new family of solar cells based on nanocrystalline and conducting polymer films is emerging, some of which show high solar conversion efficiencies, rivaling that of traditional Si devices.
Paragraph #2: Dye sensitized nanocrystalline solar cells (DSC) are based on the operating principle of charge separation between wide band gap oxide layers, a monolayer of charge transfer dye, and an electrolytic redox solution. Overall the DSC creates an electrical potential based on the difference in the Fermi level of electron in the oxide layer and the electrolyte solution, which out any permanent chemical reactions.
Paragraph#3: TiO2 layers are typically deposited by screen printing onto a conductive glass surface. Typical geometries of deposited oxide layers are 5-20 um thick and are have porosity between 50-65%.
Paragraph#4: New developments in DSCs are the use of a wide band gap nanocrystalline n-type semiconductor as an electron acceptor. The charge neutrality on the dye is restored by a p-type semiconductor acting as an electrolyte; cells with this advancement have shown efficiecies of over 10%.
Paragraph#5: DSCs have interesting history and corrsponance with photography and photo-electrochemistry. TiO2 is now playing the same role as silver halides, which also have large band gaps on the order of 2.7-3.2 eV, did for photography.
Paragraph#6: The first sensitization of a photo-electrode occurred shortly after 1873. It took until the 1960s to realize that the excitation of a dye molecule injecting electrons into the conduction band of an n-type semiconductor.
Paragraph#7: Titanium dioxide became the semiconductor of choice for a handful of reasons. Using a tri(bpy)carboxylate-Ru dye to chemisorb onto the surface of the a 7.1% efficiency DSC was announced in 1991. Progress has continued since with efficiency over 10% today.
Paragraph#8: The ideal photosensitizer would absorb all light below 920nm and contain contain carboxylate or phosphonate groups to graft to semiconductor oxide surface. Redox potential must be low to be regenerate from electron donation from the redox electrolyte and be stable enough to continuously undergo this reaction for about 20 years.
Paragraph#9: Research in dye chemistry focuses on absorption over the entire visible range. Optical absorption of a single monolayer is weak, unless the oxide layer becomes highly porous.
Paragraph#10: Ru and Os polypyridyl complexes show the most promise for dyes, i.e. the N3 dye. The max absorption are 518 and 380nm, emits at 750nm with the lifetime being 60ns.
Paragraph#11: N3 was surpassed in 1993 by the “black dye” a trpy complex, efficiency excedded 10.4%, and furthermore a dual system of N3 and the black dye were even better.
Paragraph#12: The black dye absorbs 100nm further into the IR than N3. However, both dyes show large overlap in the majority of absorption spectrum.
Paragraph#13: Molecular engineering of Ru complexes has increased light harvesting in the 700-900nm region. The goal is to mimic GaAs optical features, which would increase overall efficiency to 15%.
Paragraph#14: DSC operation is insensitive to temperature changes, however, competing Si solar cells see drastic decrease under raised temperature up to 60C.
Paragraph#15: Porphyrins and phthalocyanines are also of particular attention to organic dyes. Porphyrins generally lack absorption near the IR, while phthalocyanines have LUMO levels too low for electron transfer into the TiO2 conduction band.
Paragraph#16: Coumarine and polyene sensitizers have achieved up to 7.7% efficiency under full sunlight.
Paragraph#17: The combination of two complementary dyes to achieve broad visible absorption is another successful strategy. Many dye combinations need to be tested for this type of system.
Paragraph#18: Semiconductor Quantum dots show great promise due to very high extinction coefficients. These types of sensitizers are most attractive for solid-state heterojunction devices.
Paragraph#19: Mesoporous electrodes are very different than the same material in compact forms. The charge transport mechanism is still under debate in these systems, and percolation processes are under intense scrutiny. Intensity modulated impedance spectroscopy has been an effective tool to address these problems.
Paragraph#20: Future research on mesopourous electrodes will be based on spatial organization rather than random assembly. Nanorod and Nanotubes hold the most promise, this would facilitate pore diffusion and allow the junction to form with better control.
Paragraph#21: High surface area of nanocrystalline solar cells is important for performance enhancement. Several computational studies have been executed using classical force field and first principle density functional calculations to model adsorption of dyes onto TiO2 surface planes.
Paragraph#22: Synthetic efforts for sensitizers should focus on keeping one-directional flow of electrons from the redox electrolyte through the junction and into the oxide layer after photo-excitation of the sensitizer. New dyes should also form a tightly packed monolayer to block dark current.
Paragraph#23: Recent progress was made in controlling self-assembly of dye by adding guanidinium thiocyante to the electrolyte solution. This approach led to an efficiency of 10.6%.
Paragraph#24: A photovoltaic device must be stable for 20 years without performance degradation. A constituents must be under intense scrutiny, since the conductive glass and TiO2 films are essentially required four other components have received the most investigation.
Paragraph#25: The N3 dye is stable in elevated temperatures and can undergo up to 108 redox cycles without considerable loss of performance. Outstanding stability can be attributed to charge injection in a femtosecond time-scale. Short time frames do not allow other chemical mutations to occur.
Paragraph#26: Long term illumination of redox electrolytes like cyclic carbonates, such as propylene or ethylene carbonate, which were found to undergo thermally activated decarboxylation in the presence of TiO2 rendering them unsuitable for reliable use.
Paragraph#27: Highly polar nonvolatile solvents have been found to be ample replacements, such as methoxypropionitrile (MPN). MPN based electrolyte in conjugation with a surfactant ruthenium dye recently passed 1000h stability at 80C.
Paragraph#28: Room temperature molten salts have attractive stability and some based on imidazolium iodides have shown efficiency up to 6%.
Paragraph#29: Long-term light soaking has seen significant progress over the past few years. A recent 12,000h test under full illumination has recently passed, where traditional amorphous silicon fails due to the Stabler-Wronski effect.
Paragraph#30: The replacement of redox electrolyte by p-type semiconductor is a promising alternative to deal with sealing problems.
Paragraph#31: The sensitizing dye does not provide a conducting functionality; therefore it must be in direct contact with both conducting phases. It is not completely evident that the penetrating solid interface will allow contact intimate enough for reasonable charge transfer to occur.
Paragraph#32: The system in Paragraphs 29 and 30 have been improved since 1998. Highest efficiency has been seen at 3.8% by Meng et. al with Cu(I) as a hole conductor instead of a liquid electrolyte.
Paragraph#33: Dye-Sensitized nanocystalline photovoltaic cells have shown great promise in competing with solid-state junction devices.